Research On Machined Surface Characteristics Of304Stainless Steel

To meet the demands for electric power and economic growth, our country urgently needs to develop the nuclear power projects. At present, the manufacturing technology of nuclear reactor coolant pump (NRCP) is the technological bottleneck and difficulty on the road leading to China’s autonomy in the field of nuclear power technology. The main material of NRCP is304stainless steel which works in the long-term circumstances of high temperature, high pressure and high radiation. Therefore, it is rather important and necessary to research on the machining characteristics of304stainless steel.This dissertation focuses on the machining theory and machining characteristics of304stainless steel, which include cutting temperature, tool-chip friction, cutting force, dynamic constitutive relations, residual stress and martensitic transformation on the machined surface of304stainless steel. The main contents are as follows:(1) Quasi-static and dynamic compressive property testing of304stainless steel were conducted with Split Hopkinson Pressure Bar (SHPB) apparatus and Universal Testing Machine at room temperature and high temperatures. Then the stress-strain relations of304stainless steel were obtained under the conditions of different temperatures and different strain rates. The results show that obvious strain rate hardening and thermal softening phenomena appear in dynamic testing of304stainless steel. Based on the analysis of experimental results, the Johnson-Cook constitutive model of304stainless steel was fitted.(2) An analytical model of temperature in the non-equidistant primary shear zone (PSZ) is proposed. In this model, the PSZ which is a parallel and non-equidistant zone with a certain thickness is regarded as a volume moving heat source. Moreover, the method of modified non-uniform volume moving heat source and the method of image heat source are combined. Compared with existing models, the new model not only can be used to accurately predict the temperature distribution of the PSZ but also can be used to calculate the proportion of heat for heat conduction in PSZ. Therefore, an in-depth study of cutting heat of the PSZ is made.(3) A new analytical tool-chip friction model is proposed. Compared with existing models, this model has three characteristics:firstly, the new model quantifies the thickness of the material transfer layer; secondly, the new model strictly distinguishes the cutting interface in the secondary shear zone (SSZ) from the tool-chip contacting interface; thirdly, the new model gives the expressions of shear strain rate and chip flow velocity including three friction regions. Therefore, the flow process of the chip on the tool rake face is described comprehensively. Validations and analysis indicate that this analytical model can be used rapidly and efficiently to predict friction characteristics including the length of the sticking region, the tool-chip contact length, the global friction coefficient and the local friction coefficient on the tool rake face during machining. The results show that the sticking friction region tends to vanish and only sliding friction region remains on the rake face when the cutting velocity reaches a certain greater value in the cutting process.(4) On the basis of the thermo-mechanical coupling analysis incorporating the non-equidistant PSZ and the triple-regional SSZ, the prediction model of cutting force is given. This analytical model is validated by the experiments of cutting forces of304stainless steel and the results show that predictions are in good agreement with experiments. In addition, the effect of processing parameters on cutting forces of304stainless steel is also investigated.(5) An analytical model of residual stress distribution for machined surfaces of304stainless steel is presented. In this model, the tool edge roundness and the tool flank wear are taken into account simultaneously. The mechanical stress of machined workpiece is calculated by the superposition principle including the case of the perfectly sharp tool, the case of the tool edge roundness and the case of the tool flank wear. Based on the integration of mechanical stress and thermal stress, the principles of elastoplastic loading and unloading are utilized to predict the residual stress distribution of machined workpiece. The results show that when the roundness of the cutting dege increases in the range of0.1mm to0.4mm, the maximum residual compressive stress will increase and extend deeper below the machined surface layer for304stainless steel. And the residual compressive stress of304stainless steel increases with the increasing of the length of the tool flank wear.(6) Due to X-ray diffraction experiments of machined surfaces for304stainless steel, the quantitative phase analysis of machined surfaces is made and the effect of machining on martensite transformation for304stainless steel is summarized. Therefore, the relationship between machining parameters and martensite transformation for304stainless steel is established. The results show that part of austenite is transformed into α-martensite and ε-martensite during machining of304stainless steel. And the volume fraction of ε-martensite is obviously less than the volume fraction of α-martensite. Furthermore, it is found that turning speed, cutting thickness and cutting feed greatly influence martensite transformation for304stainless steel. Because of the special working conditions of304stainless steel in the nuclear power field, it is the suggestion that higher turning speed (>910r/min), lower cutting thickness (<0.1mm) and rational cutting feed should be chosen during machining in order to prevent machining-induced martensite transformation of304stainless steel.